1. Field of the Invention
This invention relates to the control system for a lubricating oil solvent dewaxing apparatus. In particular, the invention is a system which controls the temperature of waxy oil in the apparatus, to cause the growth of wax crystals of a uniform and moderate size.
2. Description of Related Methods in the Field
It is known in the art to dewax waxy petroleum oil stocks by cooling oil-solvent solutions at uniformly slow rates, of e.g. 1.degree. to 8.degree. F./min., (0.56.degree. to 4.4.degree. C./min.), under controlled conditions for crystallization of wax from said solutions. Commercially, such oil-solvent solutions are cooled by several methods such as indirect heat exchange in scraped surface exchangers; dilution chilling wherein waxy oil stock is contacted in a multistage tower with chilled solvent under conditions of high levels of agitation (U. S. Pat. No. 3,773,650); and direct chilling, wherein a low boiling solvent, e.g. propylene, mixed with waxy oil stock is vaporized under conditions of reduced pressure.
In such commercial processes, the waxy oil charge, or solutions of waxy oil charge and solvent, are heated to a temperature at which the wax is dissolved. The heated charge is then passed into a cooling zone wherein cooling is undertaken at a uniform slow rate in the range of about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min.) until a temperature is reached at which a substantial portion of the wax is crystallized and at which dewaxed oil product has a selected pour point temperature. Upon achieving the desired dewaxing temperature, the mixture of wax crystals, oil and solvent is subjected to solid-liquid separation for recovery of a wax free oil-solvent solution and a solid wax containing a minor proportion of oil (slack-wax). The separated oil-solvent solution is subjected to distillation for recovery of a solvent fraction and a dewaxed oil product fraction. The slack wax may be recovered as is, or may be subjected to additional processing, such as repulp filtration for removal of additional oil therefrom.
Solid-liquid separation techniques which may be employed for separation of wax crystals from the oil-solvent solutions include known solid-liquid separation processes, such as gravity settling, centrifugation, and filtration. Most commonly, in commercial processes, filtration in a rotary vacuum filter, followed by solvent wash of the wax cake, is employed.
Dewaxing solvents which may be used in solvent dewaxing processes include known dewaxing solvents. Commonly used solvents include aliphatic ketones of 3-6 carbon atoms, C.sub.2 -C.sub.4 range hydrocarbons, C.sub.6 -C.sub.7 aromatic hydrocarbons, halogenated C.sub.1 -C.sub.4 hydrocarbons, an mixtures of such solvents.
Solvents known to be useful as dewaxing solvents are the ketones containing 3 to 6 carbon atoms, for example, acetone, methylethylketone (MEK) and methylisobutylketone (MIBK); mixtures of ketones; and mixtures of ketones with aromatic hydrocarbons including benzene and toluene. Halogenated low molecular weight hydrocarbons, including dichloromethane and dichloroethane, and their mixtures are also known dewaxing solvents. Solvent dilution of wax oil stocks maintains fluidity of the oil for facilitating easy handling, for obtaining optimum wax-oil separation, and for obtaining optimum dewaxed oil yields. The extent of solvent dilution depends upon the particular oil stocks and solvents used, the approach to filtration temperature in the cooling zone and the desired final ratio of solvent to oil in the separation zone.
For processes employing indirect cooling in scraped surface exchangers, cooling and wax crystallization are accomplished under conditions of very little agitation at a cooling rate in the range of about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min. Under such conditions, without wall scrapers, wax tends to accumulate on the cold exchanger walls, interfering with heat transfer and causing increased pressure drop. Thus, wall scrapers are employed to remove the accumulated wax. Dewaxing solvents are employed to maintain fluidity of the oil in the coolers, and may be added before the oil is cooled or in increments during cooling. Often the oil is given a final dilution with solvent at the separation temperature for reducing solution viscosity such that wax separation is more efficient. Commonly, solvent added to the oil in such processes is at the same temperature, or somewhat higher temperature than the oil. Cold solvent, added at substantially lower temperatures than the oil, shock chills the oil resulting in formation of many small wax crystals which can be difficult to separate. Under controlled conditions, elongated wax crystals of good size are formed which are easy to separate and which contain little occluded oil.
Dilution chilling processes employ incremental addition of cold solvent, e.g. to +20.degree. to -25.degree. (-6.7.degree. to -32.degree. C.) to the oil at high degrees of agitation such that oil and solvent are completely mixed in less than one second. Under such conditions, wax precipitates in small, hard balls rather than elongated crystals. Such wax precipitates are easy to separate and retain very little oil.
Direct chilling processes employ a low boiling hydrocarbon, e.g. propylene, as dewaxing solvent and refrigerant. Waxy oil stock is diluted with sufficient low boiling hydrocarbon to provide the necessary cooling and provide the desired final dilution for separation of solid-wax from the oil-solvent solution. The low boiling hydrocarbon is vaporized from the oil-low boiling hydrocarbon solution, under conditions of reduced pressure, at a rate sufficient to cool the solution about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min). Such cooling is continued until the desired separation temperature and amount of wax crystallization are obtained. At the separation temperature, sufficient low boiling hydrocarbon remains in solution with the oil to provide the desired fluidity for separation of wax. Agitation of the mixture being cooled is commonly provided for reduction of temperature and concentration gradients.
U. S. Pat. No. 3,972,779 to C. W. Harrison teaches a system for controlling a dewaxing apparatus. The system comprises a viscosity measurement of the waxy oil-wax crystal slurry mix and a refractive index measurement of the solvent to calculate incremental changes to dilution of wax oil with solvent.
U. S. Pat. No. 4,375,403 to C. W. Harrison et al. teaches a solvent dewaxing process employing direct heat exchange cooling of a solvent-oil mixture by direct heat exchange with an immiscible liquid chilling medium. The immiscible liquid may be any liquid which is substantially inert with respect to the solvent and oil stock, e.g. ethylene glycol.
U. S. Pat. No. 4,354,921 to H. J. Pitman et al. teaches a solvent dewaxing process in which a portion of the oil-solvent mixture is chilled to a temperature above its cloud point and a further portion is chilled to a temperature below its cloud point. The portions are combined to produce a mixture substantially at the cloud point. The combined mixture is further cooled to dewaxing temperature at which wax crystals are separated from the solvent and refined oil.
U.S. Pat. No. 3,764,517 to Bodemuller, Jr. teaches a control system for a solvent dewaxing process. Waxy oil is first heated to a temperature above the melting point of the wax. Cold solvent is admixed with the heated oil to shock chill the waxy oil-solvent mixture by direct heat exchange to a temperature below the wax melting point and above the wax crystallization temperature in the waxy oil-solvent mixture. The shock chilled oil solvent mixture is then chilled at a final chilling rate of 0.3.degree. F./min. to 3.0.degree. F./min. by indirect heat exchange to a filtration temperature about 10.degree. F. below the desire pour point of the dewaxed oil product and filtered. A signal is generated corresponding to the filtrate flow rate and the shock chilling solvent temperature controlled in response to the signal.
Petroleum Refiner, Vol. 15, No. 6, June 1936, pp. 205-209 discusses the useful art of solvent dewaxing and the terminology used by those trained in the art.